Silicon deposition in diatoms: control by the pH inside the silicon deposition vesicle

To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brébisson et Kützing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kützing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3-(2,4-dinitroanilino)-3'-amino-N-methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion

Molecular chemical aspects of silica gel formation

This chapter describes some molecular chemical aspects of the formation of silica gels from aqueous silicate solutions. Experiments involved silicon-29 nuclear magnetic resonance spectroscopy and small-angle scattering of X-rays. Cations are shown to significantly affect the dissolution and oligomerization processes responsible for the formation of silica gels. Aging of aqueous silica gels is described in terms of the changes in fractal dimensionality of the silica structure. Preliminary SAXS experiments on gel transformations occurring in zeolite A synthesis mixtures indicate large-scale changes in gel morphology before crystalline zeolite formation occurs

Growth and silica conten of the diatoms Thalassiosira weissflogii and Navicula salinarum at different salinities and enrichments with aluminium

The dependence of the cellular (biogenic) and frustule-associated (mineralized) silica content of the diatoms Navicula salinarum and Thalassiosira weissflogii on salinity and aluminium conditions was studied in order to make it possible to manipulate silicification in vitro and maximize it to levels required for physico-chemical frustule characterization by physisorption, X-ray scattering analysis and NMR, which is our ultimate objective. Enrichments with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T. weissflogii. Aluminium additions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silica matrix of the frustule. In contrast, lowering the salinity from 28 practical salinity units (PSU) to 20 and 15 resulted in a significant increase in the biogenic silica content per cell of both species, which is in line with an increase in density of the chemically derived silica under low salt conditions. The silica content per cell was variable during culture growth; increase in cell densities (during exponential growth) was accompanied by a decrease in contents of biogenic silica per cell. Electron microscopy and energy dispersive X-ray analysis (EDXA) support the chemical analyses and also suggest higher biogenic silica contents at lower salinities. The results indicate that besides silicate concentration and pH, the concentration of salts is an important inorganic factor that affects the silica polymerization inside the silica deposition vesicle of diatoms

Diatom silicon biomineralization as an inspirational source of new approaches to silica production

The demand for new materials and products is still growing and the interest in naturally formed biopolymers and biominerals, such as chitin, calcium precipitates and silica is increasing. Photosynthesizing microalgae of the family Bacillariophyceae (diatoms) produce silica exoskeletons with a potential to be used in specific industrial or technological processes, they also are an excellent model in studies of silicon biomineralization. In contrast to geologically aged diatomaceous earth, the freshly prepared silica of cultured or harvested natural diatoms has been characterized insufficiently with respect to the properties (e.g. purity, specific surface area, porosity) required for technological and industrial application. In this contribution we summarize aspects of cellular processes that are involved in silicon biomineralization of diatoms and the current knowledge of the characterization of diatomaceous silica, following methods used for synthetically derived silica-based materials

Томский диалектный корпус как новая ветвь изучения особенностей говоров Среднего Приобья (на примере морфологической разметки)

\u3cp\u3eSilica syntheses have performed by acidification of water glass under various reaction conditions and analysed bmeans of SAXS method. SAXS spectra recorded during simultaneous dosing of water glass and sulpharic acid have shown clearly that primery SiO\u3csub\u3e2\u3c/sub\u3e particles grow continously and rapidly during the precipitation. In addition, the primery particles have a rather narrow size distribution, what is suprising result, since the particles are continuously involved in a number of processes: growing, forming interparticle bonds and aging.\u3c/p\u3

Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle X-ray scattering study

Combined small and wide angle IZ-ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO2 matrix of diatom frustules, One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra, These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d-value approximate to 0.06 mn) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of -4.0 to -2.5, with two clear common regions among species: d <10 nm (slopes 4, denoted as region I and also called the Pored region), and 10.0 <d <40.0 nm (slopes -2.9 to -3.8, denoted as region LT), The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO2 matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl3 to the growth medium, The results indicate that diatoms deposit silica with pore

The influence of cations on growth kinetics of silica aggregates

Silica gels are prepd. by acidification (pH = 2-4) of water glass. SAXS measurements show that the gel consists of fractal aggregates (D = 2.2). Although the fractal dimension is not influenced by addn. of cations, Al3+ and Mg2+ retard the growth of the fractal aggregates, while TMA has a promoting effec

Aggregation, gelation and aging in silica

A review on the combination of fractal concepts with exptl. data obtained by non-invasive methods (NMR, SAXS, SANS, visible light scattering) for studying silica dispersions. Significant progress has been made in understanding elementary processes involved in the aggregation, gelation, and aging steps during the prepn. of porous silica. 29 Refs. [on SciFinder (R)